A distinction is made between passive and active transponders. In the case of a passive transponder, the supply of power and the data interchange between the transponder and the reader are effected using magnetic or electromagnetic fields. In this case, the transponder uses an antenna to pick up the power required for operation from the field of the reader. In contrast to this, the active transponder has its own power source, for example a battery. The active transponder can modulate either passively using load modulation or actively using a driver stage.
For the contactless data transmission between a transponder and a reader, it is important to observe a precise clock. In conventional systems, for example based on the ISO/IEC 14443 communication standard, with a passive transponder, the clock (in the ISO/IEC 14443 communication standard: 13.56 MHz) for operating the transponder both in the transmission mode and in the reception mode is derived from the clock of the reader. The transmission mode is usually effected by means of load modulation, this type of modulation allowing the clock to be derived easily.
If such a clock signal is not available to the transponder in the transmission Mode-which may be the case with active transponders, for example-then the transponder requires its own clock generator.
A conventional transponder may have its own oscillator. In this case, the oscillator is coupled to the signal received from the reader in a phase-locked manner by means of a phase-locked loop (PLL) circuit when the transponder unit is in the reception mode. When the transponder unit is in the transmission mode, that is to say during data transmission from the transponder unit to the reader, the control voltage from the PLL circuit is kept constant. Clock synchronization between the reader and the transponder unit does not take place during this time. An attempt is merely made to keep the frequency of the oscillator as stable as possible. Particularly when transmission times in the transponder unit are relatively long, this can result in clock discrepancies and hence in a disturbance in the data transmission.